The field of electrochemical sensors is one which occupies continuing research and commercial interest, for both industrial and medical applications. One class of sensor which shows considerable promise is the sort disclosed in U.S. Pat. No. 4,020,830 to Johnson et al. entitled "SELECTIVE CHEMICAL SENSITIVE FET TRANSDUCERS". The Johnson et al. patent describes a field effect device wherein a variably conductive channel extends between respective drain and source regions, with the drain to source current being modulated by a chemically selective system which overlays the channel region. In particular, the chemically selective system often includes an insulator layer, overlayed by a chemically selective membrane which reacts selectively to the ambient substance to be monitored. A reference electrode, suitably biased relative to the device, is coupled to the monitored material to facilitate selective interaction of the material to be monitored with the membrane system, thereby in turn modulating the drain to source current.
It has become popular in the art to classify chemically sensitive devices of the sort disclosed in the Johnson et al. patent by the nature of the chemically selective system, and the ambient materials with which they react.
For example those reacting with ions are often designated "ISFETS", those reacting on an immunological basis are "IMFETS", and so on. As utilized herein, the term "chemfet" shall designate devices of the sort described, irrespective of the nature of the reaction between the chemically selective system of the device, and the designated ambient material. Likewise, the "chemfet" designation shall be utilized for such devices whether or not they utilize separate insulator and membrane overlayments, or simple insulator areas which acquire an oxidation layer or the like while in use, or a variety of other chemically selective systems. Further, the term shall designate unspecified applicable semi-conductor devices, such as field effective devices, Schottky devices, and so on.
A common prior approach to utilization of chemfet devices is to maintain the device substrate at ground potential, to maintain the drain region at a specified DC potential, and to maintain the source electrode at some fixed potential relative to the drain, e.g., ground potential or virtual ground through an operational amplifier system. The reference electrode is maintained at a specified potential relative to the device (e.g., relative to the source electrode). Typically, such a configuration (i.e. "DC mode operation") operates for known devices at potentials in the range of 2 volts, DC reference electrode current in the range of 100 picoamps, and drain to source currents in the order of 100 to 500 microamperes.
Such operating constraints afford no difficulty for industrial or even in vitro medical applications, but provide potential problems for in vivo medical applications. For example, it is contemplated that chemfet devices may be utilized as the active element of an invasive intravenous or tissue catheter-type sensor, wherein the device directly serves to measure physiological parameters such as blood ion concentrations, blood gas tension, or the presence of immunological agents in the body. Generally, the chemfet device utilized in such application will be enclosed by a highly insulating, hermetically sealing material, except that the gate membrane area which overlies the drain to source conductive channel will be open to the ambient materials. In the event that the encapsulation should fail, or that there should occur a breach of the insulator/membrane system, there is an immediate and substantial risk of harm to the patient. In particular, such a fault would expose the patient to DC currents of 10 to 100 times the levels normally considered safe. Further, since the body of the patient would thereby become a portion of the circuit, conventional current monitoring techniques would be difficult to apply, and would in any event be subject to uncertainty regarding the actual path of the currents so generated.
It is, accordingly, an object of the present invention to provide safer modes of operation for chemfet devices, particularly addressing safety problems associated with direct in vivo applications of the devices. It is an associated object to provide such systems and apparatus which are essentially intrinsically safe by virtue of their operation, and which do not require current or voltage sensing followed by open or closed loop corrections and/or alarms.